The use actively-controlled switches such as transistors, usually power MOSFETs or power BJTs, to replace diodes is a technique for improving the efficiency of rectification in power supplies.
In low voltage converters, for example around 10 volts and less, the voltage drop of a silicon rectifying diode (typically around 0.7 to 1 volt for a silicon diode at its rated current) has an adverse effect on efficiency. One solution known in the art is to replace standard silicon diodes with Schottky diodes, which exhibit very low voltage drops (as low as 0.5 volts). However, even Schottky rectifiers can be exhibit significant losses, notably at high currents and low voltages.
In power supplies using synchronous rectification, power switches replace the rectification diodes and are driven synchronously with the waveform to be rectified so that the switch is conducting during a portion of the waveform and blocking during the remaining portion of the waveform.
Several methods for driving synchronous rectifiers are known in the art. The basic categorization involves directly driven methods and self-driven methods.
With the directly driven methods, the driving signal is generated by an integrated controller device typically circuitry contained in an integrated circuit chip. The cost of the integrated controllers may be significant.
With the self-driven methods, the driving signal is generated using discrete components and extra transformer windings which produce the necessary signals for driving the synchronous rectifying devices.
By way of example of an existing solution, referring to FIG. 1 there may be seen an electrical schematic diagram of a power supply having self-driven synchronous rectifying elements. Primary input voltage is provided at 101 to one side of the primary winding 103 of power transformer 102. Coupled to the other side of primary winding 103 is capacitor 107 and first primary drive field effect transistor (FET) 109. Second primary drive FET transistor 108 is connected to the other side of capacitor 107 and then to the other primary input voltage leads 111. First primary FET transistor also connects to the other primary input voltage lead 111. Gate drive for FET transistors 108 and 109 is provided at 110 and 112 respectively and consists of appropriate out-of-phase drive signals which alternately turn on and off FETs 108 and 109 to produce an alternating current in primary winding 103 of transformer 102.
Secondary winding 104 of transformer 102 has two synchronous rectifying FET transistors, FET 114 and FET 115 which are driven by circuitry described below so as to conduct at appropriate times to rectify the voltage waveform produced across secondary winding 104. Filter inductor 113 and filter capacitor 116 act to smooth voltage and current variations in the output current and voltage respectively. Resistor 118 and capacitor 117 are illustrative of loads on the power supply, while a secondary side ground reference point may be seen at 119a. 
Tertiary windings 105 and 106 of transformer 102 serve to produce the driving voltages for the synchronous rectifying FETs 114 and 115. Voltage pulses produced at windings 105 and 106 due to the variations in current in the primary winding 103 are passed through capacitors 120 and 123 respectively, to the gates of FETs 114 and 115 respectively. Diodes 121 and 124 serve to provide a current path during the reverse voltage cycles of windings 105 and 106, while resistors 122 and 125 ensure that the gates of FETs 114 and 115 will be turned off when no driving voltage is present. A secondary side ground reference point may be seen at 119b, and in this example is conductively continuous with 119a. 
This solution may contain several disadvantageous operational characteristics. Firstly, during the converter start-up operation, the synchronous rectifier driving circuit can pull current out from the circuit load. Since the magnitude of this current for the driving is essentially uncontrollable, the current flowing from the circuit load represents significant risk of component failures on both sides, i.e. on the side of power converter and on the side of circuit load. In the industry, this problem is frequently referred to as the pre-biased output start-up problem. Secondly, during the converter shut-down operation, the driving circuit may generate driving voltages which can exceed the synchronous rectifier ratings introducing risk of the rectifiers and overall power converter failure.
In view of the foregoing, it would be desirable to provide a means for enabling such a synchronous rectifier power supply to operate reliably and fault-free during start-up and shut-down transients.